Tryptophan and histidine residues of native bovine parathyroid hormone (PTH) and the corresponding synthetic biologically active 1-34 amino acid sequence will be modified chemically. Biological activity and immunological reactivity of the resulting derivatives will be compared with the original material in order to elucidate the specific biological roles of the appropriate amino acid residues. In assessing the data, consideration will be given to the potential role of conformational changes which may result from the labelling of the residues in question.